After all this talk about building test models and towing rigs, the three people still reading this are probably wondering just how it all turned out.
Now that she's actually hit the water, we know: The model Almaguin behaves exactly as expected. Whether lightly loaded or filled to capacity, she showed no signs of yawing problems, no signs of porpoising, no chine walking, and no other undesirable behaviours. Bow rise during the climb to plane was a bit less than expected- that's good. Slamming into a head sea at a scale speed of 16 knots shipped only a few drops of spray (mainly from the tow boat's wake) and didn't upset the boat at all.
The model's towed from a soft tether at her centre of gravity, and so is free to porpoise, chine-walk or yaw as much as she likes. The bow and stern lines (included to keep her pointing straight when we dock the big boat, and in case things get out of control) are flapping in the breeze, but are not tight and are not putting much if any force on the boat.
But enough talk, let's let embedded Youtube do the work.
For the first run, she's loaded to 3.7 kg, for a scale displacement of 237 kg, equivalent to running empty with just a driver. At 1/4 scale, the scale speed is exactly double the true speed; when the dial reads 15 mph, the model's effectively doing 30 mph / 26 knots. This is about as fast as one would ever have reason to go in an Almaguin- you could probably push her higher (we took the model to 36 mph / 31 knots scale, with no ill effects), but it'd take a skilled driver and calm conditions to travel at these speeds.
For the second run, we'll load her to 7.5 kg (scaled, her rated all-up limit of 480 kg). Once again, we'll take her up to 30 mph / 26 knots. The real boat can't quite reach this speed when fully loaded, if she's properly powered- an engine large enough to push a half-tonne boat to the 30 knot mark would make her squirrely and hard to control when she's running light. The model's throwing up quite a bit more spray here- not surprising, since there's twice as much boat in the water- but almost none of it comes aboard, and the ride is smooth and predictable through the entire speed range.
Next: Using the tow boat to create a 20 cm high wake (scale: 80 cm, or a three-footer) and having the model take it on the starboard bow at a scale speed of 16 knots. You would not want to try this in the real boat; it will be uncomfortable and is a pretty intense test of your skill with fibreglass filleting and of your plywood supplier's quality control. But the boat stays in control, and what little spray does come aboard is from the tow boat's bow spray.
A more detailed analysis with higher resolution pictures is coming in a few days. For now, suffice it to say that I'm satisfied the Almaguin design performs, at model scale, exactly as intended.